"Forwarding ports" means setting up a router so that packets sent from the internet to your IP address at any of the forwarded ports will reach your VOIPo adapter, assuming that your router is between the internet and your VOIPo adapter. If you don't have a router between the internet and your VOIPo adapter, there is nothing to forward.
If I'm not mistaken, all three of the devices in your diagram can serve as routers. You don't need to forward any ports on the router portion of your VOIPo adapter, because it knows what to do with the SIP packets when it receives them. You don't need to forward any ports on your WRTU54, because it is not between the internet and your VOIPo adapter. However, if your Actiontec DSL modem is set up as a router (as well as a DSL modem), and if it is performing Network Address Translation (NAT) for multiple devices on your network, then you may need to forward ports, because packets may not be getting to their proper destination.
However, there are many variables. How is the Actiontec set up? Is it in bridge mode? Does it perform the PPPoE login? Does it handle DHCP? These are things that a support rep can walk you through in order to figure the right way to set it up.
If you don't get someone over the weekend, you might just try connecting the WAN port of the VOIPo adapter to one of the LAN ports of the WRTU54 to see what happens.
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